This invention relates generally to measurement devices and more particularly to calorimetric devices for the measurement of radio frequency power.
As is known in the art, radio frequency (RF) power detectors operating on calorimetric principles are called calorimeters and are used typically in making high power RF measurements. Because most RF power detectors are incapable of dissipating power levels exceeding several hundred watts of average or pulsed power, calorimeter detectors are often used to provide a measurement of the incident RF power by converting the incident RF energy into thermal energy and equating the level of thermal energy to a known equivalent RF power level.
One type of measurement device widely used for measuring high RF power signals is the waterload. In using a waterload, the high level RF power is absorbed in a matched-impedance section of a transmission line that is wholly or partially filled with a flowing stream of liquid, usually water. It is generally desired that the liquid itself dissipate all of the incident power, so that the temperature difference between the liquid entering the load and the liquid leaving the load can be measured by calibrated thermocouples, thermistors, or thermometers. One of several techniques can then be used for relating the desired absolute RF power to the corresponding rise in temperature.
In one type of waterload, a water filled chamber is disposed at an end portion of a housing and input and output ports are provided to the chamber such that, a pressurized water supply is provided to the chamber. In this way, cool water is provided to the load through the input port and the heated water is removed from the chamber from the output port.
Mathematical expressions have been derived for relating a conversion of RF power to heat, and the levels of the RF power in the measurement device into thermal energy. One basic approach which may be used in conjunction with waterloads is discussed below.
As is known by those of ordinary skill in the art, the thermal capacity (c) of a substance is the number of calories (cal) needed to raise one gram (g) of the substance through a temperature change of one degree centigrade (C). The thermal capacity of water is 1 gal/g..degree.C. Therefore, in order to raise the temperature of a mass (m) having a thermal capacity (c) through a temperature change (.DELTA.T), the quantity (Q) of heat (in units of calories) required is: EQU Q=mc.DELTA.T
where c is the factor which relates the thermal capacity of the substance of mass m to the thermal capacity of water (c=1.0).
To determine the flow rate (f) of a fluid mass, the above relationship can be expressed as: ##EQU1##
Flow rates are generally expressed in units of cubic centimeters per second (cc/sec) or in gallons per minute (gpm). Since a calorie is equivalent to 4.186 joules, a watt is equal to a joule/sec, and a cubic centimeter of water is equal to a gram of water, the flow rate (f) in units of gallons per minute provides an expression for dissipated power (P) as: ##EQU2##
Accordingly, a direct relationship of measured power to temperature rise of the fluid at a given flow rate can be established.
The accuracy of the power measurement is dependant, to a large extent, on the ability of the waterload to absorb all of the incident RF energy and to transfer that energy into the fluid mass of the water passing through the absorbent element of the load. Energy either reflected away from the load or dissipated as heat external to the water will contribute to the inaccuracy of the measurement. For this reason, it is desired that the waterload have a voltage standing wave ratio (VSWR) characteristic that is as low as possible.
It is generally desirable that the measurement device or system used in making the measurement be calibrated so that the accuracy of the measurement can be relied upon.
One method of verifying the measurement accuracy of a waterload is to provide a power regulated heater element within the path of the water exiting the waterload for converting electrical power into heat. In other words, the necessary amount of power needed to raise the temperature of the water a desired amount, is the same at D.C. or any other frequency. For example, 100 watts of power at a frequency of 60 Hz, that is applied to the power regulated heater element will provide the same rise in temperature to the water as 100 watts of microwave power provided to the waterload assuming the frequency of the microwave power is within the operating bandwidth of the waterload.
A calorimetric measurement system, using a calorimetric waterload, generally will include the above-mentioned power regulated heater coupled to one of the pipes or hoses provided to the absorbent element of the waterload and a pair of temperature measurement devices, such as thermocouples, thermistors, or thermometers. One of the temperature measurement devices is disposed along the fluid supply line and the other along the fluid return line. The temperature measurement devices measure a temperature of the water entering and exiting the waterload to provide the reference delta temperature needed for calibration. The calorimetric measurement system will also generally include an automated calorimetric system controller or a DIGI-CAL calorimetric system. A DIGI-CAL system generally includes a housing for storing the necessary control instrumentation, such as the power regulator, as well as display readouts, front panel controls and interface buses needed for making automated computer measurements. The DIGI-CAL controller also generally includes a fluid storage tank required for containing the water supplied to the calorimetric waterload. One such DIGI-CAL calorimetric system controller is the Model No. ASTC-3000A-1, manufactured by the Raytheon Corporation, Waltham, Mass.
A typical calibration procedure for such a calorimetric system includes the supply of pressurized water to the calorimetric waterload at a desired flow rate until the system has been purged of any air trapped in the flow path. Power is applied to the power regulated heater, disposed to the outlet path of the waterload, at a level in the general range of the RF power expected to be measured. For example, if a transmitting source is expected to deliver 1 KW of RF power, 1 KW of power is applied to the regulated heater. When the DIGI-CAL display indicates that the system has reached a steady state condition, the temperature difference between the inlet and outlet paths to the waterload are sensed and sent to the DIGI-CAL to generate a reference measurement based on the direct heating of the liquid exiting the waterload. The power regulated heater is periodically referenced to a calibrated wattmeter which can be traced to and referenced with a National Institute of Standards and Technology (NIST) standard. The RF or microwave power signal is then applied to the waterload and an indication of the power measurement is observed at the DIGI-CAL controller.
However, for calorimeter measurements involving very high levels of microwave power (&gt;1 KW), the power regulated heater used for calibration will also require the equivalent level of power. Providing such a high power system of this type is generally very expensive and can easily have a cost that exceeds the cost of the calorimeter system. Furthermore, providing power levels at these levels generally will provide a safety hazard to operators using the equipment. Moreover, because the heater element is generally required to be driven with a well regulated current source, the cost of such regulators can be expensive and will significantly increase the cost of the system.